The present invention relates to a switch mechanism; and more particularly, it relates to a switch mechanism which is responsive to two or more displacement-type actuators. The term "displacement actuator" is intended to cover a broad class of mechanical actuators or transducers which sense pressure or force and convert it to mechanical movement or displacement such that the displacement is proportional to the parameter being sensed or monitored (pressure or force, for example). Mechanical linkage may be used to amplify the displacement which is then coupled to a toggle mechanism of a switch for tripping the switch if the displacement exceeds an amount which represents a predetermined threshold for the parameter being monitored.
Devices of this type find a wide range of application. For example, they are used in refrigeration or cooling systems in which one displacement actuator is responsive to the suction pressure of a compressor and a second displacement actuator is responsive to the output pressure of the compressor. It may be desired to shut the compressor off if the suction pressure falls below a predetermined low threshold (which may indicate loss of refrigerant or oil or a leak in the system), or if the outlet pressure exceeds a predetermined high threshold (which would present too great a load on the compressor motor). In this example, a switch would have a set of main contacts and one or more sets of auxiliary contacts. If either alarm condition sensed by the transducers occurred, it would be desired to open the main contacts and thereby de-energize the compressor motor, and to close the auxiliary contacts to indicate that an alarm condition exists.
A problem exists in conventional switch mechanisms of this type which contain two toggle mechanisms each responsive to a different displacement actuator and mechanically interconnected to open the main contacts when either sensing threshold or "set point" is exceeded in that each toggle mechanism itself typically includes a spring (for establishing bistable conditions in the switch mechanism) and some movement (which may be the output of the displacement actuator) is required to trip the toggle mechanism. If the toggle mechanisms are mechanically interconnected or connected in series, the outputs of each actuator has a tendency to interfere with and thereby change the threshold of the other toggle mechanism.
This is particularly true when the switches are used in refrigeration or cooling systems because the outputs of the displacement actuators change dynamically when the system is in operation. For example, once the compressor motor is energized, the output pressure increases while the suction pressure decreases simultaneously. Hence, if there is any mechanical interference or interrelation, the movement of one displacement actuator may vary the actual trip point of the toggle mechanism associated with the other displacement actuator, and this type of interference or interdependency has been found to exist in some present commercial structures.
Briefly, the present invention overcomes this problem by providing a switch which has two independent toggle mechanisms, each responsive to a different displacement actuator. In the illustrated embodiment, a set of main contacts are normally closed, and two sets of auxiliary contacts are normally open. One toggle mechanism is adapted by means of a first sliding yoke to actuate a movable contact of the main set and a movable contact of a first auxiliary set of contacts. The other toggle mechanism is similarly adapted by means of a second, independent sliding yoke to actuate the movable contact of the second auxiliary set and the movable contact of the main set. Each yoke has a shoulder which engages the movable main contact when the associated toggle mechanism is tripped, but the motion of the movable main contact is not reflected in the other toggle mechanism. Thus, the main contact can be moved without in any way interfering with or varying the spring forces in the other toggle mechanism.
With this structure, the normally closed contacts of the main set are opened if either pressure limit is exceeded, but the only auxiliary set that is closed is that which is associated with the tripped toggle mechanism. This auxiliary set of contacts may be used to indicate a specific alarm identifying which displacement actuator has exceeded its threshold. Either toggle mechanism, or both, may be set to trigger on a pressure rise (or increase above a set point) if a conventional reverse action linkage is used in connection with the associated displacement actuator.
The toggle mechanisms are independent in the sense that a varying pressure sensed by one of the displacement actuators does not change the spring forces of the other toggle mechanism.
Either toggle mechanism may have an automatic reset or it may have a manual reset independent of the other toggle mechanism. This is useful, for example, where it is desired to lock the system out if one threshold is exceeded (for example, the low suction pressure threshold), but simply to de-energize the compressor motor only for so long as the other sensed condition prevails (for example, only for so long as the high pressure threshold is exceeded).
Other features and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts of the various views.